A Study of 2 Doses of a New TB Vaccine, MVA85A, in Healthy Volunteers Previously Vaccinated With BCG

The dose of recombinant MVA used in the TB trials to date is relatively low compared with other trials using recombinant MVAs which have used up to 2.5 x 108pfu (A Hill, personal communication). Having demonstrated safety and immunogenicity of 5 x 107pfu of MVA85A, we now need to perform a dose optimization study, prior to commencing larger scale Phase II and III studies in South Africa. We will vaccinate 12 volunteers with a dose half a log lower than the dose we are currently using, i.e. 107pfu MVA85A, and 12 volunteers with a dose half a log higher, i.e. 108pfu.

MVA85A (at a dose of 5 x 107pfu) has been administered to 41 healthy volunteers in the UK and 17 healthy volunteers in The Gambia, with no serious adverse events. We have designed our Phase I studies to allow for a vaccination of volunteer groups sequentially with a step-wise increase in mycobacterial exposure, in order to minimize the possibility of a Koch reaction. A Koch reaction describes the development of immunopathology in a person or animal with tuberculosis, when an exaggerated immune response to M.tb is stimulated. It was described in patients with TB disease when Koch performed his original studies employing mycobacteria as a type of therapeutic vaccination. It has now been demonstrated in the mouse model of therapeutic vaccination (Taylor, 2003). Available animal data suggest that these reactions do not occur in mice latently infected with M.tb, suggesting that such reactions may correlate with high bacterial load and that the Koch phenomenon may not pose a problem for vaccination of healthy albeit latently infected humans. We started these studies in healthy volunteers who were as mycobacterially naïve as possible. They were skin test negative and Elispot negative for PPD, ESAT 6 and CFP10, and had not had previously been vaccinated with BCG. We have now completed studies in the UK vaccinating volunteers previously vaccinated with BCG (McShane, 2004). These volunteers are excluded if their Mantoux test is greater than 15 millimeters. These studies are ongoing in The Gambia. The group we are currently recruiting for on this increasing mycobacterial spectrum are healthy volunteers who are latently infected with M.tb.

Decision matrix for selecting MVA85A dose

1. If reducing the dose of MVA85A results in reduced immunogenicity, we could use either the 5x107 or 108 dose, and would select the lower of these on the grounds of safety. 2. If increasing the dose of MVA85A increases the incidence of side-effects, we could use either the 5x107 or 107 dose, and we would use the higher of these on the grounds of immunogenicity. 3. If increasing the dose of MVA85A results in increased immunogenicity and no increase in side-effects, we could use either the 5x107 or 108 dose in subsequent trials 4. If reducing the dose of MVA85A results in no reduction in immunogenicity we could use either the 107 or 5x107 dose in subsequent trials 5. If all three doses are equally safe and immunogenic we will use either the low or 5x107 dose.